1. Field of the Invention
The present invention relates to a hologram recording/reproducing device and a hologram reproducing device.
2. Description of the Related Art
A hologram recording device and a hologram recording method, used for recording data using holography, are proposed. In the device and method, reference light and signal light, modulated by information to be recorded (record data), are generated by the same laser light source, and are used to irradiate a hologram recording/reproducing medium. This causes the signal light and the reference light to interfere with each other at the hologram recording/reproducing medium to form a diffraction grating (hologram) in the hologram recording/reproducing medium, so that the record data is recorded in the form of the diffraction grating (hologram).
A hologram reproducing device and a hologram reproducing method, used for reproducing the record data from the diffraction grating (hologram) recorded in this way, are proposed. In the device and method, when the diffraction grating (hologram) formed on the recording medium on which the recording has been performed is irradiated with reference light, diffraction light (that is, reproduction light) is generated. The reproduction light is detected by a light-receiving element to reproduce the record data.
Two types of recording/reproducing methods are proposed for generating the signal light and the reference light in such a recording operation and a reproducing operation. They are a two-beam interference recording/reproducing method and a collinear recording/reproducing method. In the two-beam interference recording/reproducing method, a path of the signal light and a path of the reference light are completely separately provided, whereas, in the collinear recording/reproducing method, the signal light and the reference light are disposed on the same axis and share one path. In the collinear recording/reproducing method, a reference-light pattern is formed at an outer peripheral portion of a spatial-light modulator (SLM) and a signal-light pattern is formed at an inner peripheral portion of the spatial-light modulator for recording on a recording medium. In addition, only the reference-light pattern is formed at the outer peripheral portion of the spatial-light modulator, and reproduction light is obtained from the recording medium on which the recording has been performed to reproduce record data (refer to, for example, U.S. Pat. No. 6,108,110 and Nikkei Electronics (P106 to 114 of the Jan. 17, 2005 issue).
A recording/reproducing optical unit 150, which is a main portion of a recording/reproducing device that performs a recording operation and a reproducing operation, is shown in FIG. 7. A transmissive collinear recording/reproducing method will be simply described with reference to the recording/reproducing optical unit 150 shown in FIG. 7.
Information is recorded in the following way. A light beam emitted from a laser light source 101 for recording and reproducing the information is incident upon a spatial-light modulator 103 through a collimator lens 102. The spatial-light modulator 103 spatially divides the light beam into light-beam portions, one portion becoming signal light 108 having a light-intensity modulation pattern reflecting record information and the other portion becoming reference light 109 which is not subjected to light intensity modulation or which is subjected to a particular light intensity modulation. These portions of the light beam reach an objective lens 104. That is, the collimator lens 102, the spatial-light modulator 103, and the objective lens 104 are disposed in a forward path that is a light-beam path extending from the laser light source 101 to a hologram recording/reproducing medium 307. For the spatial-light modulator 103, for example, a combination of a polarizing plate and an array liquid crystal panel is used. Since the signal light 108 and the reference light 109 pass through the same objective lens 104, the recording/reproducing method is called the collinear recording/reproducing method. The objective lens 104 causes the signal light 108 and the reference light 109 to form an interference fringe, that is, a hologram, at an information recording layer in the transmissive hologram recording/reproducing medium 307.
The information is reproduced in the following way. A light beam emitted from the laser light source 101 is transmitted through the collimator lens 102 and reaches the spatial-light modulator 103. The signal light 108 generated from the light beam is blocked by the spatial-light modulator 103 whose transmissivity is controlled to 0%, so that only the reference light 109 illuminates the recorded hologram through the objective lens 104. The light beam that has been diffracted by the hologram in the transmissive hologram recording/reproducing medium 307 passes through an objective lens 105 to form a reproduction image on an array light detector 106, where an array light-receiving element is disposed, and to detect a spatial distribution of light intensity of the reproduction image by the array light detector 106. Here, the array light detector 106 is, for example, a CCD sensor or a C-MOS sensor. The objective lens 105 is disposed in a backward path that is a light-beam path extending from the hologram recording/reproducing medium 307 to the spatial-light modulator 103.
In the collinear recording/reproducing method, the recording/reproducing optical unit may be a reflective type. FIG. 8 shows an example of a structure of a reflective recording/reproducing optical unit 151. Parts of the optical unit 151 having structural features and functions that are similar to those of the optical unit 150 shown in FIG. 7 will be given the same reference numerals and will not be described below. In a reflective collinear recording/reproducing method, a reflective hologram recording/reproducing medium 207 having a reflective film at the back of an information recording layer is used. Recording of information in the reflective type is substantially the same as that in the transmissive type. A difference is that signal light 108 and reference light 109 pass through a polarization beam splitter 110 and are converted into circularly-polarized lights by a ¼ wavelength plate 111 to form a hologram at the information recording layer in the hologram recording/reproducing medium 207 by an objective lens 104. The information is reproduced as follows. A light beam that has been diffracted and reflected by the hologram in the hologram recording/reproducing medium 207 passes through the objective lens 104, is converted into linearly-polarized light that is perpendicular to a forward path by the ¼ wavelength plate 111, and is reflected by the polarization beam splitter 110, so that a hologram image is formed on an array light detector 106 and a spatial distribution of light intensity of the reproduction image is detected by the array light detector 106. That is, a collimator lens 102, a spatial-light modulator 103, the polarization beam splitter 110, the ¼ wavelength plate 111, and the objective lens 104 are disposed in the forward path that is a light-beam path extending from the laser light source 101 to the hologram recording/reproducing medium 207. The objective lens 104, the ¼ wavelength plate 111, and the polarization beam splitter 110 are disposed in the backward path that is a light-beam path extending from the hologram recording/reproducing medium 207 to the spatial-light modulator 103. A recording/reproducing optical unit that does not use the ¼ wavelength plate 111 and that uses a beam splitter, instead of the polarization beam splitter 110, is known.
FIG. 9 shows an example of a pattern disposed at the spatial-light modulator 103 for splitting the signal light 108 and the reference light 109 transmitted through the spatial-light modulator 103 of the hologram recording/reproducing device. In general, a signal-light area 118 for generating the signal light 108 is disposed at an inner peripheral portion of the spatial-light modulator 103 where good optical performance is provided, a reference-light area 119 for generating the reference light 109 is disposed at an outer peripheral portion of the spatial-light modulator 103, and a gap is provided between the signal-light area 118 and the reference-light area 119.
To record a large amount of information on a hologram recording/reproducing medium, what is called multiplex recording for forming a plurality of holograms at the same location (or overlapping areas) of the hologram recording/reproducing medium may be performed. Various multiplex recording methods are proposed (refer to, for example, Nikkei Electronics (P106 to 114 of the Jan. 17, 2005 issue)).
In such collinear hologram recording/reproducing devices, in performing a reproducing operation, when the reference light is guided to the light detector, the reference light that is scattered in the backward path with respect to a hologram reproduction image having a very low light quantity becomes noise in the hologram reproduction image, thereby preventing stable hologram signal detection.
Accordingly, it is desirable to provide a hologram recording/reproducing device and a hologram reproducing device which make it possible to overcome the aforementioned problems to prevent reference light from being guided to a light detector and scattered reference light in a backward path from becoming noise in a hologram reproduction image.